The present invention is directed to xe2x80x9cactive implantable medical devicesxe2x80x9d such as those defined by the Jun. 20, 1990 directive 90/385/EEC of the European Community Council, and more particularly, to cardiac pacemakers and/or defibrillators, having a function that is enslaved to, i.e. responsive to, a parameter sensed by a sensor. Although the following description refers mainly to the case of a cardiac pacemaker, the invention also is applicable in a general manner to a great variety of electronic devices having a control function that is responsive to a sensed parameter.
Active implantable medical devices are known to adapt their actions, for example, a control function such as the stimulation frequency in the case of a cardiac pacemaker, to the measured or calculated value of a parameter representative of the metabolic needs (cardiac output requirements) of the patient bearing the device.
EP 0 750 920 (and corresponding U.S. Pat. No. 5 722 996, which are both commonly owned by the assignee hereof, Ela Medical, and which are both incorporated herein by reference) describes the utilization of two sensors in a cardiac pacemaker, an activity sensor and an effort sensor, and proposes particularly a method to select the most rapid (in terms of response) sensor at the onset and at the end of an effort exerted by the patient, and the physiological sensor during the effort.
A xe2x80x9ccross controlxe2x80x9d takes place to the extent that one sensor alone determining an effort, unconfirmed by the second, will not be taken into account except during a predetermined time (typically 15 seconds for the activity sensor and 2 minutes for the effort sensor). During an effort confirmed by both of the two sensors, enslavement will be based solely on the effort sensor (that is to say the sensor taken into account preferentially during the effort is the effort sensor) without control by the activity sensor.
According to the basic principle of enslavement set forth in EP 0 750 920 (which is implemented in the xe2x80x9cTALENT 213xe2x80x9d pacemaker available from ELA MEDICAL, the assignee hereof), an escape interval is calculated for the activity sensor every 1.5625 seconds which corresponds to new information of the sensor (i.e. update or refresh rate of the sensor information). Every four cardiac cycles, the former is averaged on the last four escape intervals. The escape interval is calculated for the effort sensor every respiratory cycle. Every four cardiac cycles, the former is averaged on the last eight escape intervals.
Values of the escape interval of the effort sensor and the activity sensor allow determination of states of the sensors. The two sensors calculate independently an escape interval. From the evolution of escape intervals, a state is defined for each sensor. There are three states:
Given that there are two sensors and three states for each sensor, there are nine (in combination) states possible. For each (combination) state, the pacemaker adopts a different behavior. Each combination determines a value of the command escape interval, that is to say the escape interval to which the period of enslavement is tied. While enslavement with double sensors, such as that described previously and the object of the EP 0 750 920, is mainly based on the information provided by the effort sensor, it can happen that the escape interval defined by this sensor is not in concordance with the level of activity of the patient, in the case, for example, of significant movements. In this case, the command escape interval and the period of enslavement can have values not corresponding to the needs of the patient, being either too low or too high.
One of the objects of the present invention is to solve the foregoing problems and to define control of the command escape interval in the case where it is equal to the escape interval defined by the effort sensor. The present invention is directed to an active implantable medical device capable of enslaved functioning to at least one physiological parameter.
The device of the present invention comprises at least one effort sensor for measuring a primarily physiological parameter and delivering a signal which is a function of the effort exerted by a patient bearing the device, at least one activity sensor for measuring a primarily physical parameter and delivering a signal which is indicative of activity of the patient bearing the device, means for analyzing periodically the relative sequence of successive state changes of the sensors according to predetermined criteria intrinsic to the device, means for determining a command escape interval controlling the enslaved functioning of the device from the analyzed sensor states, and means for limiting the command escape interval to predetermined values.
According to one embodiment, limitations of the command escape interval are a function of the activity sensor escape interval.
According to another embodiment, limitations of the command escape interval correspond to the subtraction of a predefined constant percentage of the activity sensor escape interval.
Preferably, one applies two limitations, a lower limit and an upper limit, to the command escape interval.
Additionally, in an advantageous manner, the lower and upper limits are (re)defined each time that the means for analysis of the states of the sensors determines that the activity sensor escape interval is modified.
According to another embodiment, the means for limiting the command escape interval apply the limitation to the command escape interval only where the effort sensor is in a state other than the rest state.
Preferably, the means for limiting the command escape interval limits the enslavement frequency where the effort sensor detects an effort not detected by the activity sensor.